1. Field of the Invention
The present invention relates to modified amino compounds, preferably imidazole compounds, which are used as curing agents for epoxy resins, to methods of curing epoxy resins, and to cured epoxy resins produced by such methods.
2. Description of Related Art.
Epoxy resins are relatively low molecular weight linear polymers which may be cured (crosslinked) to form higher molecular weight thermoset resins useful in applications such as adhesives, abrasion resistant coatings, molding compounds and similar applications. Epoxy resins, such as the reaction product of bisphenol A and epichlorohydrin, may be cured by the reaction of a polyfunctional amine or alcohol with the terminal vicinal epoxide groups, or through the reaction of pendant hydroxyl groups present along the epoxy polymer chain with a polyfunctional acid or acid anhydride, resulting in the formation of ester linkages. Other materials containing active hydrogen atoms are also known curing agents for epoxy resins, such as polyamines, polyamide resins, phenolic, urea and melamine resins, and the like.
Amino compounds, such as imidazole compounds, are known in the art as curing agents for epoxy resins. Epoxy resins cured using certain imidazoles such as imidazole itself or alkyl substituted imidazoles are found to exhibit highly desirable physical and chemical properties. However, the water solubility of such compounds may result in the leaching out of unreacted imidazole (either during the process of curing or during final use) causing environmental problems.
One approach to improving the curing properties of imidazoles is to modify them by blending or reacting them with other materials. For example, U.S. Pat. No. 4,066,625 discloses a modified imidazole curing agent prepared by first reacting an imidazole compound with an epoxy resin, which reaction product is subsequently mixed with additional epoxy resin to form a crosslinked polymer system. However, the patent teaches that the resin must be heated to temperatures in excess of 100.degree. C., for example 140.degree. C. or 160.degree. C., to effect a proper cure. Similarly, U.S. Pat. Nos. 4,358,571, 4,420,605 and European Patent 24119 teach imidazoles modified by the addition of a C.sub.1 to C.sub.18 alkyl acrylate, by an epoxy material or by carbamoylation, followed by neutralization, as curing agents for epoxy resins. Once again, however, relatively high curing temperatures well in excess of 100.degree. C. are used to crosslink the epoxy resin. German patent document DE 2318754 teaches the preparation of fast curing epoxy resins (18-20 seconds at 180.degree. C.) cured with modified imidazoles prepared by reacting an imidizole compound with an organic anhydride such as chlorendic anhydride or tetrahydrophthalic anhydride. Other patents disclosing similar modified imidazoles are Japanese patent document JP51-129,600 (imidazole modified by reaction with an isocyanate group) and Japanese patent document JP58-052,819 (imidazole modified by reaction with an epoxy compound and a copolymer obtained by polymerizing an alkenyl phenol and an alkene cyanide).
Yet another approach known in the art for preparing modified imidazole epoxy curing agents is to incorporate imidazole compounds into a non-epoxy polymeric backbone either by reaction with terminal functional groups present in the polymer or by copolymerization. For example, Japanese patent disclosure JP54-164,499 discloses epoxy curing agents prepared by the reaction of 2 moles of an imidizole compound with a carboxy terminated liquid rubber such as polybutadiene, ABN rubber, ABS rubber and similar materials. However, because the concentration of such end-capped curing agents is limited by the low concentration of end groups, auxiliary curing agents are often required to give a satisfactory cure. Japanese patent document JP 62-141,519 discloses a similar curing agent prepared by reacting a liquid ABN rubber end-capped with a compound which may include an amine having one or more active hydrogens. Other prior art references teach incorporating ethylenically unsaturated imidazole monomers, such as vinylimidazoles or imidazolyl methyl styrene, into polymeric backbones by copolymerizing the imidazole monomer with at least one other copolymerizable monomer. Such references include Japanese patent documents JP 56-115,313 and JP 57-119,921 and U.S. Pat. Nos. 4,349,651 and 4,430,445.
While the above approaches to providing curing agents for epoxy resins may be suitable for various applications, there exists an objective in the art to provide epoxy curing agents and epoxy resins cured using such agents which are curable at relatively low temperatures below about 100.degree. C. For example, where the epoxy resins are used in applications such as liners or interior coatings for in-place pipes (water pipes, sewerage pipes and the like) it is most desirable to cure the resin within the damp pipe in-situ after it has been applied to the pipe interior. This may be readily accomplished by the passage of hot water through the pipe if the epoxy can be cured at the temperature of the water which is, of course, below 100.degree. C., normally about 75.degree. C.
It is also important in such applications that the epoxy curing agent not have a tendency to leach out of either from the uncured resin during the casting in a wet underground pipe, or from the cured epoxy during continual uses, for environmental purposes, that it be essentially water insoluble and that the cured resin possesses good mechanical properties such as tensile and flexural strength.